In a small heavy saucepan combine the cream and brown sugar, bring the mixture to a boil over moderately-high heat, whisking occasionally, and whisk it until the brown sugar is dissolved.

Remove the pan from the heat, add the bittersweet chocolate and the unsweetened chocolate, and whisk the mixture until the chocolate is melted. Whisk in the butter until the sauce is smooth and let the sauce cool slightly.

The chocolate sauce may be made 1 week in advance and kept covered and chilled.

Reheat the chocolate sauce over very low heat, stirring occasionally, until it is warm.